1. Field of the Invention
The invention relates to a firing mechanism for a gun utilized to effect the perforation of a subterranean well casing or liner through the application of a fluid pressure force to the firing mechanism.
2. Description of the Prior Art
Perforating guns have long been employed to effect the perforation of a well casing or liner in the vicinity of a production zone and to produce fractures extending into such production zone. The popular perforating guns now uniformly employ the so-called "shaped charges", which are disposed in vertically and angularly spaced relationship relative to the casing axis so as to produce a large number of evenly spaced perforations with a single firing. Such shaped charges are generally ignited by a primer cord which contacts a primer end of each shaped charge container to detonate the charge contained within each such container.
The ignition of the primer cord is commonly accomplished by dropping a weight or bar on an impact actuated primer or detonator which is stationarily positioned immediately above the perforating gun housing and is operatively connected to the primer cord which extends downwardly through the perforating gun housing. The employment of a detonating bar dropped through a tubular conduit as a means for effecting the ignition and discharge of the perforating gun has encounttered difficulties in those wells wherein the well bore deviates substantially from the true vertical when passing through a particular production zone. The deviation may be sufficiently great that the fall speed of the detonating bar is substantially reduced, to the point that insufficient impact energy is imparted to the primer charge to effect its discharge. In other wells where it is necessary to employ a high-density kill fluid or the like, the existence of such fluid in the conduit bore, through which the the detonating bar is dropped, can very well reduce the speed of the detonating bar to an ineffective level. Debris collecting around the hammer can have the same effect. Any failure to detonate the primer charge obviously imposes a substantial cost and time penalty on the completion of the well.
It has previously been proposed that the actuation of the hammer to fire the primer charge be effected by fluid pressure forces establishing a differential between the fluid pressure existing in the tubing string carrying the perforating gun versus the fluid pressure existing in the annulus around such tubing string, but when utilizing such fluid pressures to effect the actuation of the hammer of the perforating gun, another factor must be taken into account, namely the desirability of perforating the well with the fluid pressure existing in the casing annulus adjacent the zone to be perforated being maintained at a relatively low level so that the perforating of the adjoining production formation is accomplished with the well in the so-called "underbalanced" condition. In other words, the anticipated fluid pressure of fluid from the perforated production zone should desirably be in substantially excess of the fluid pressure existing within the well casing at the moment that perforation is accomplished, so that flow from the peforated production zone can immediately commence with a substantial velocity and thus remove the debris naturally associated with the perforating operation from the perforations in the production formation.
Such underbalanced condition is commonly produced by suspending the perforating apparatus from a tubing-carried packer which is set at a position above the production zone to be perforated. A cross-over sub is then provided to divert the fluid transmitted by the tubing bore into the casing annulus below the packer for the reason that it is much easier to reduce the fluid pressure existing in the annulus adjacent the production zone to be perforated by a swabbing operation in the tubing string. The annulus fluid pressure existing above the packer is transmitted to the firing mechanism for the perforating gun by an axially extending fluid passage provided in the packer. Thus an increase in annulus pressure accompanied by a simultaneous decrease in tubing pressure will produce a pressure differential sufficient to cause the fluid pressure actuation of the hammer of the firing mechanism, while at the same time producing an underbalanced condition in the casing annulus adjacent the production zone to be perforated. A fluid pressure actuated firing mechanism for a perforating gun of this general type is disclosed and claimed in co-pending application, Ser. No. 593,396, filed Mar. 26, 1984, and assigned to the Assignee of the present invention now U.S. Pat. No. 4,594,335.
A fluid pressure actuating mechanism for a perforating gun which is operable solely by changes in the selected fluid pressure would obviously provide a simplified system for effecting the fluid pressure actuation of a well perforating gun.